The goals of this research are to characterize how experimental, clinical or traumatic alterations of the muscles of mastication modify (1) the intrinsic properties of the masticatory muscles themselves and (2) the overall morphology and function of the craniofacial region. Specifically, this research examines adaptations of the masticatory muscles to changes in their chronic length and to increases in activity, decreases in activity, immobilization and injury. The results from these studies are important for understanding the biology of how the masticatory muscles adapt to altered function and morphology. In addition, they bear direct relevance to understanding the impact of orthodontic and surgical procedures and traumatic events on subsequent form and function of the masticatory apparatus and the craniofacial region. This research contains five sets of related experiments that examine (1) the effects of chronic masticatory muscle lengthening in rhesus monkeys on regional adaptations in electromyographic activity of the muscles of mastication. The remaining experiments study adaptations in the physiology and morphology of the muscles of mastication in rats following (2) chronic increases in length of the masticatory muscles; (3) increased functional load of the masseter muscle following myectomy of the temporalis muscle; (4) reduced function resulting from immobilization of the mandible or an altered diet; and (5) trauma resulting from myotomy of the masseter muscle. Methods of study include electromyography in the primate experiments; and isometric contractile properties analysis, stimulated bite force, and morphology, histology and histochemistry of muscle and connective tissues in the studies on rats.